Motor control system for motorized ophthalmic instrument

ABSTRACT

An improved refractor for use in the subjective examination of human eyes. Each half of the refractor includes a plurality of lens supporting disks, a hub assembly for rotatably supporting the disks, and a comb assembly to space the disks along the optical axis. One of the disks supports at least one set of 4 cross cylinder lenses. Each disk is coupled to a stepping motor for selective rotation. Each motor is, in turn, supported by a bracket which permits rotation equivalent to a partial motor step to insure alignment of the optical elements supported on the disks with the optical axis. The cylinder lenses supported on the disks are also rotated by a stepping motor. The Risley prisms of each half of the refractor are always positioned on the optical axis and rotated by stepping motors. Finally, the refractor includes electronics, including a microprocessor, to control the rotation of the motors as required to move the various optical components as necessary to emulate the ophthalmic prescription of the person whose eyes are being examined.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following cofiled applications: Ser.No. 285,715 of D. E. Stevens for Motorized Refraction Apparatus; andSer. No. 285,717 of Peter Augusto et al. for ANDed Motor Control Systemfor Motorized Ophthalmic Instrument.

DESCRIPTION OF THE PRIOR ART

There are a wide variety of refracting instruments used in the clinicalpractice of ophthalmology and optometry, including: conventionalrefractors, automated monocular objective refracting devices andautomated monocular and binocular subjective refracting devices.

A conventional refractor consists of a pair of housings in which arepositioned corrective optics for emulating the ophthalmic prescriptionrequired to correct the vision of the patient whose eyes are beingexamined. Typically, each housing contains sets of spherical andcylindrical lenses mounted in rotatable disks. The two housings aresuspended from a stand or wall bracket for positioning in front of thepatient's eyes. Further, in front of each refractor housing a number ofaccessories are mounted, typically on arms, so that they may be swunginto place before the patient's eyes. Typically, these accessoriesinclude a variable power prism known as a Risley prism, Maddox rods, anda cross cylinder for performing the Jackson cross cylinder test.

In determining a patient's distance prescription, the patient views avariety of alpha numeric characters of different sizes through variouscombinations of the spherical and/or cylindrical lenses supported in therefractor housings until the correct prescription is emulated. Thecharacters, which are typically positioned 6 meters away, may be on achart or may be projected on a screen by an acuity projector. For nearvision testing the same procedure is repeated, expect that the alphanumeric characters viewed by the patient are positioned on a bracket 20to 65 centimeters in front of the refractor housing.

The cross cylinder is used to refine the power and axis position of thecylindrical component of the patient's prescription. The cross cylinderis a lens consisting of equal power plus and minus cylinders with theiraxes 90 degrees apart. It is mounted in a loupe for rotation about aflip axis which is midway between the plus and minus axes. When thecross cylinder is flipped, the plus and minus axes change places.

In the Jackson cross cylinder test, the patient views a target throughthe spherical and/or cylindrical lenses of the refractor used to emulatethe patient's prescription. The cross cylinder is used by lining up itsflip axis with the previously determined astigmatism correcting cylinderaxis. When the cross cylinder is flipped, if each of its positionsproduces an equal blur of the target, the astigmatism correctingcylinder axis is proper. If one position is clearer than the other, theastigmatism correcting cylinder axis is rotated toward the crosscylinder axis which makes vision better. The process is continued untilan equal blurring is achieved when the cross cylinder is flipped. Then,to check cylinder power, the cross cylinder is rotated 45 degrees,thereby bringing one of its axes parallel with the correcting cylinderaxis. The cross cylinder is again flipped, and equal impairment ofvision indicates the correct cylinder power. When the astigmatismcorrecting cylinder is negative and if vision is better with the minusaxis of the cross cylinder parallel to the correcting cylinder axis, thecylinder power should be increased, and vice versa. These steps arerepeated until equal impairment is observed in each position.

To insure that the flip axis of the cross cylinder is aligned with thepreviously determined astigmatism correcting cylinder axis andthereafter to maintain the cross cylinder flip axis in coincidence withthe cylinder axis through the usual numerous corrections to the cylinderaxis, the cross cylinder mechanism is mechanically coupled to thecylinder lenses. U.S. Pat. No. 3,498,699 discloses a cross cylinderloupe assembly mechanically coupled to correcting cylinder lenses inorder to maintain proper orientation of the cross cylinder assembly.U.S. Pat. No. 3,860,330 also describes a mechanism for synchronizing theaxial orientation of a cross cylinder lens assembly with the cylinderaxis of a correcting cylinder lens.

In the above described mechanisms, the cross cylinder is placed in theoptical path of the refractor only after the prescription has beeninitially determined. U.S. Pat. No. 4,185,896 discloses a refractorcross cylinder mechanism in which a pair of cylinder lenses are alwaysin the optical path of each refractor half. Each pair of cylinder lenseshas a combined power of zero when their cylinder axes are parallel and asmall cross cylinder power when one lens is rotated until its axis isperpendicular to the cylinder axis of the other lens.

A Risley prism is a "rotary prism" used for finding the necessaryprismatic correction of a patient's eye. It consists of two ophthalmicprisms of equal power, one in front of the other, and mounted so thatthe prisms can be rotated about the optical axis of the refractor half.In the initial position the base of one prism corresponds with the edgeof the other, so that the two prisms are equivalent to a glass platewith plane parallel faces. The maximum effect is obtained when the basesof the prisms correspond.

As those skilled in the art will appreciate, in order to proceed withthe subjective determination of a patient's refractive error, it isnecessary to have a starting point. Typically, this is accomplished byan objective examination of a patient's eyes, through variouscombinations of the spherical and/or cylindrical lenses supported in therefractor housings, with a retinoscope. This procedure, particularly fora previously unrefracted patient, can be quite time consuming. To reducethe time required to make an objective measurement of a patient'srefractive power, a number of objective automatic monocular objectiverefraction devices, also known as automatic infrared optometers, havebeen developed. Several of these devices are described in and comparedwith other refracting instruments in Clincial Ophthalmology, Volume 1,Chapter 67, "Automated Clinical Refraction", D. L. Guyton, Thomas D.Duane (Editor), Harper & Row, 1980.

A similar instrument developed by Zeiss, and the subject of U.S. Pat.No. 3,791,719, includes a "refractometer attachment" in combination witha motorized refractor. It is stated that the lens disks are rotated byservomotors and that micro switches are used to accurately limit therotary movement of these motors. In operation, the refractometerattachment delivers signals, corresponding to the state of refraction ofthe eye, to the servomotors to move one or more of the lenses supportedon the lens disks into the optical path to achieve a rough refraction.The apparatus also includes a control unit for manually actuating theservomotors after switching off the automatically operatingrefractometer. The manually operated control unit is used to moveselected corrective lenses in front of the patient's eyes for subjectiverefraction.

Automated subjective refracting devices include American OpticalCorporation's SR III and SR IV subjective refraction systems, HumphreyInstruments' Vision Analyzer, and H. Schwind's Refraktron. The SR IIIand IV, based on the subjective optometer disclosed in U.S. Pat. No.3,664,631, uses axially moveable lenses to achieve continuously variablespherocylindric power over a wide range. In operation, the patient looksinto the instrument and focuses or aligns a programmed series of specialline targets. Like the automatic infrared optometers, these instrumentsare intended to provide a preliminary refraction that is usuallysubsequently refined by the practitioner.

The Humphrey Instruments' Vision Analyzer, disclosed in U.S. Pat. No.3,874,774, is designed to perform the entire binocular refraction, bothdistance and near, and thus replace the conventional refractor. Theinstrument includes a projection system in which pairs of variablelenses are incorporated. Light from the targets is collimated, passedthrough the variable-power lenses, deflected by mirrors designed forinterpupillary distance adjustment, and finally collected by a concaveviewing mirror located approximately 3 meters away from the patient. Theconcave mirror reimages the variable-power optics directly in front ofthe patient's eyes. Because the optics are reimaged in front of thepatient's eyes, the target appears to be located on the mirror. Whileintended to replace the conventional refractor, conventional refractioncannot be performed with this instrument. Thus, the starting point forthe subjective refraction must be obtained from the patient's priorprescription or an objective refractor. Also, because of the opticaldesign, conventional subjective refraction techniques cannot be used.

The Schwind eye testing instrument includes a refractor havingconventional batteries of spherical and cylindrical lenses, an opticalsystem for projecting a series of vision testing slides and asemi-reflective mirror. In operation the patient whose eyes are to beexamined is seated in front of the semi-reflective mirror. A blackboardor other similar surface may be placed on the opposite side to provide anon-distracting background. A target image is projected, via a suitableoptics, through various combinations of the spherical and/or cylindricallenses of the refractor halves for viewing by the patient. Though notapparent to the patient, the optical effect is as though the lenses ofthe refractor were in front of the patient's eyes. As with the VisionAnalyzer, conventional objective refraction cannot be performed with theinstrument, but must be made with other instrumentation.

In the chapter on Automated Clincial Refraction, D. L. Guyton describesa computer actuated refractor as follows: "Marg et al (24) have taken amore direct approach by developing a computerized system to performsubjective refractions using conventional refractor optics andconventional refracting techniques. The most recent model, Refractor III(FIG. 67-16), is a specially designed binocular refractor containing afull range of trial lenses and accessory optical devices for each eye.The spherical and cylindric lenses, cross cylinders, prisms, Maddoxrods, filters, and pinhole apertures are arranged on four disks withineach half of the refractor. The disks are driven by stepping motors inresponse to commands from the computer. The computer instructs thepatient by means of tape-recorded of voice-synthesized messages andpresents a variety of slides for visual acuity determination andrefraction at both distance and near, using random-access slideprojectors. The patient responds with a simple push-button box held inhis lap as the computer follows a series of flow charts to arrive at therefractive correction and corrected visual acuity."

SUMMARY OF THE INVENTION

An ophthalmic instrument having a base and one or more optical elementspositionable along an optical axis thereof. The instrument includes atleast one disk having a plurality of openings for supporting at leastsome of the optical elements. The disk is supported relative to the basefor rotation about an axis from a position where one of the openings isaligned with the optical axis to, at least, one other position whereanother opening is aligned with the optical axis. One of the positionson the disk constitutes a reset or zero position when aligned with theoptical axis. The instrument also includes, for each disk, a steppingmotor having a plurality of detent positions. Each motor is in one ofits detent positions when the disk to which it is coupled is in itsreset position. The instrument further includes apparatus for sendingsignals to each of the motors and apparatus for interrupting thosesignals before the detent position at which the associated disk is inits reset position, but subsequent to the immediately preceding detentposition. The apparatus for sending signals to each of the motorsinclude a microprocessor and a motor driver which keeps track of whichphases of the motor are on and which are off and, in response to signalsfrom the microprocessor, sequentially changes which phases are on andwhich are off. For each motor, the apparatus for interrupting thesignals thereto includes a reflective block on the associated disk, anassociated encoder secured to the base, a decoder logic and a latch. Theencoder, which includes an LED and a phototransistor, sends analogsignals to the decoder logic, which analog signals change when the lightemitted by the LED is reflected back to the phototransistor by theblock. The decoder logic sends a digital signal to the latch, whichsignal changes when the analog signals from the phototransistor reach athreshold value. The latch latches as function of the change in digitalsignal and sends a signal to the microprocessor that one of the latcheshas been triggered. The latch also sends a signal to a memory, coupledto the microprocessor, identifying which latch has been triggered. Toinsure proper triggering of the encoder, apparatus is provided whichpermits lateral adjustment thereof in the plane of the associated disk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the one of refractor halves of the presentinvention;

FIG. 2 is an enlarged partial section of the center support for the lensdisks taken along Section A--A of FIG. 1;

FIG. 3 is a side view of one of the perpherial supports for the lensdisks taken along Section B--B of FIG. 1;

FIG. 4 is a perspective view of another of the peripherial supports forthe lens supporting disks;

FIG. 5 is a partial sectional view of one of the lens disk drive motorsand its associated support;

FIG. 6 is a top view of the refractor half of FIG. 1 with the lens disksremoved;

FIG. 7 is a partial sectional view of the Risley prism mechanism takenalong Section C--C of FIG. 6;

FIG. 8 is a schematic view of the auxiliary disk showing how the crosscylinders are mounted thereon; and

FIG. 9 is a block diagram of the control, drive and display electronicsof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates, in top plane view, the right refractor half 11 withits cover (not shown) removed. Refractor half 11 is one of two halves ofa refractor, such as schematically illustrated in U.S. Pat. No.4,395,097. Refractor half 11 includes a back plate 13 on which issupported rotatable disk assembly 15 and motor drive assemblies 17, 19,21, 23 and 25. Back plate 13 includes a cutout 27 through which variouswiring harnesses (not shown) pass. Back plate 13 also includes a lip 29against which the cover seats.

As best illustrated in FIG. 2, rotatable disk assembly 15 includes fivelens supporting disks: strong sphere lens disk 31; strong cylinder lensdisk 33; weak sphere lens disk 35; weak cylinder lens disk 37; andauxiliary disk 39. Lens mount 41, disk 31, is one of a series of mountsfor supporting the series of strong spherical lenses 42-52, illustratedin FIG. 1. Typically, these lenses range in 4 diopter steps from -28.00Dto +16.00D. Similarly, rotatable lens mount 53, is one of a series ofmounts in strong cylinder lens disk 33 for supporting a series of strongcylinder lenses (not shown). Depending upon the refraction technique tobe used, the cylinder lenses will range, typically, in 2 diopter stepsfrom either -2.00D to -6.00D or from +2.00D to +6.00D. Lens mount 55, indisk 35, is for supporting one of a series of weak spherical lenses (notshown), ranging in 1/4 diopter steps from 0.00D to +3.75D. Rotatablelens mount 57 is for supporting one of a series of weak cylindricallenses (not shown). Again, depending upon the refraction technique to beused, these lenses will range, typically, in 1/4 diopter steps fromeither -.25D to -1.75D or from +.25D to +1.75D. Finally, rotatable lensmount 59, in auxiliary disk 39, is for supporting the cross cylinderlenses of the present invention as well as a series of well knownauxiliary elements (also not shown) such as a pinhole, occluder, filtersand Maddox rods. At least one aperture in each of disks 33-39 remainsopen. In normal operation, a plano lens 60 is in the "open" aperture ofdisk 31 to provide additional glass thickness to optimize the opticalpath length when no strong sphere lens is required.

For the various lenses and auxiliary elements supported on disks 31-39to be accurately positionable on and along optical axis 61 of refractorhalf 11, it is necessary that disks 31-39 be accurately positionedrelative to rotational axis 63. In the present invention, this isaccomplished by radially supporting disks 31-39 with hub assembly 65, asillustrated in FIG. 2. The proper spacing of disks 31-39 along axis 63is affected by comb assemblies 71, 73 and 75, as shown in FIGS. 1, 3 and4.

Hub assembly 65 includes a disk shaft 77, a drive tube 79 and spools 81,83, 85, 87, 89 and 91. Disk shaft 77 includes an elongated cylindricalbearing surface 93, threads 95, a lip 97, and a screw slot 99. Threads95 are received in the threaded opening 101 provided in boss 103 whichis, preferably, formed as an interval part of back plate 13. Boss 103also includes a surface 105 and a tapped opening 107, as illustrated inFIG. 2. Drive tube 79 includes a bore 109 in which surface 93 of shaft77 is received. Lip 97 of shaft 77 is received within cutout 111 andshaft 77 is tightened until rear surface 113 of drive tube 79 engagessurface 105 of boss 103, and then backed off to allow drive tube 79 torotate about shaft 77. A soft plastic ball 117 received within tappedopening 107 is forced against threads 95 by a set screw 119 to preventfurther rotation of disk shaft 77.

Supported on surface 121 of drive tube 79 and captured between lip 123and bowed lock ring 125, received in groove 127, are spools 81-91. Spool81 includes an internal cylindrical bore 131, which snuggly fits oversurface 121, cylindrical bearing surface 133 and surface 135.Identically, spool 83 includes bore 139 and surfaces 141 and 143; spool85, bore 145 and surfaces 147 and 149; spool 87, bore 151 and surfaces153 and 155; spool 89, bore 157 and surfaces 159 and 161; and spool 91,bore 163 and surfaces 165 and 167. Spools 81-91 are keyed to drive tube79 so as to prevent relative rotation.

As is also evident from inspection of FIG. 2, each of disks 31-39includes, respectively, bearing apertures 171, 173, 175, 177 and 179. Toradially align disks 31-39 relative to axis 63, disk 31 is fitted oversurface 133; disk 33, over surface 141; disk 35, over surface 147; disk37, over surface 153; and disk 39, over surface 159.

The positioning of disks 31, 33, 35, 37 and 39 along axis 61 isaccomplished by comb assemblies 71, 73 and 75, illustrated in FIGS. 1, 3and 4. With reference to FIG. 3, comb assembly 71 comprises an uprightpost 181 secured to boss 183 of back plate 13 by, preferably, a bolt(not shown). Post 181 is provided with five grooves 185, 187, 189, 191and 193 which are spaced relative to each other so as to hold lens disks31-39 at those intervals along optical axis 61 which will result in therequired optical element air spacing between the lenses supported onlens disks 31-39. Each of grooves 185-193 is dimensioned so as toprovide a bearing fit for its respective disk. Since, in the preferredembodiment disks 31-39 are stamped out of 0.050 inch thick sheet metaland are ordinarily flat within 0.005 inches, post 181 is positionedadjacent optical axis 61 to insure that the required optical air spacingis maintained along axis 61.

As illustrated in FIG. 4, comb assembly 73 includes an L-shaped supportbracket 201, the short leg 203 of which is secured, via bolts (notshown), to boss 205 provided on back plate 13. Secured to upstanding leg207 is fork member 209 which includes a base portion 211, an offsetportion 213, and a pair of fork members 215 and 217. Base portion 211 issecured to leg 207 via screws 221 and 223 which pass through enlargedholes (not shown) in legs 207 and 211 and are received in a Tinnermantype nut plate (also not shown). The enlarged holes permit both heightand angular adjustment. Fork member 215, like post 181 of comb assembly71, is provided with five equally spaced grooves, of which 225 and 227are illustrated in FIG. 4. These grooves, also dimensioned so as toprovide a bearing fit, in association with grooves 185-193 of combassembly 71 and an identical set of grooves (not shown) provided on combassembly 75, position disks 31-39 along optical axis 61. Fork member 217includes no grooves.

In order to selectively rotate disks 31-39 about axis 63, to align oneor more lenses and/or auxiliary elements with optical axis 61, asrequired to emulate a patient's ophthalmic prescription, disks 31-39 arecoupled to, respectively, motor drive assemblies 17-25. With referenceto FIGS. 1 and 5, assembly 17 includes motor support bracket 241,stepping motor 243 and pinion 245. Bracket 241 includes a base 247 andtwo upstanding legs 249 and 251. Base 247 includes a pin 253 and tappedholes 255 and 257. Legs 249 and 251 include, respectively, tapped holes259 and 261. Motor 243, preferably North American Phillips K82701-P2 orequivalent, includes an integral frame bracket 263 and a drive shaft265. Pinion 245 includes a hollow hub 267 and gear teeth 269 which meshwith teeth 271 formed on the periphery of disk 31. In the preferredembodiment, the gear ratio between pinion 245 and disk 31 is chosen suchthat for every 18 steps of motor 243, disk 31 is rotated from a positionwhere one lens is aligned with axis 61 to a position where animmediately adjacent lens or opening is aligned with axis 61.

As assembled, bracket 241 is secured to back plate 13 via screws 273 and275 which pass through washers (not shown) and enlarged openings 277 and279 in bosses 281 and 283 formed in back plate 13. Pin 253 is rotatablyreceived in opening 285 of boss 287, which is also integral with backplate 13. Enlarged openings 277 and 279 permit the position of bracket241 to be angularly adjusted about the axis of pin 253. Motor framebracket 263 is secured to bracket 241 via motor attachment screws 291and 293 which pass through washers and enlarged openings therein (notshown). Pinion 245 is secured to drive shaft by means of an elastomericmaterial 295 bonded to both members.

To remove the backlash between teeth 269 and teeth 271 and to reducenoise, pinion 245 is "spring loaded" into engagement with disk 31. Withmotor attachment screws 291 and 293 loosened, motor frame bracket 263 ismoved toward axis 63 until all backlash is removed from the gear mesh.Motor frame bracket 263 is then moved an additional incremental distancetoward axis 63 and screws 291 and 293 tightened to thereby clamp motorframe bracket 263 to bracket 241. This second predetermined movementdisplaces elastic material 295 to, in effect, "spring load" gear teeth269 of pinion 245 into engagement with gear teeth 271 on disk 31.

Since the stepping motors are programmed to index only in integralsteps, the optical axes of the strong sphere lenses supported on lensdisk 31 may, within the rotational increment of disk 31 produced by onestep of motor 243, be offset from optical axis 61. To insure opticalalignment, it is necessary to provide for limited rotation of motorframe bracket 263. This angular rotation is permitted by the couplingbetween motor support bracket 241 and back plate 13. With bracketattachment screws 273 and 275 loosened, motor 243 is energized to holdit in one of its magnetic detent positions. Motor 243 and bracket 241are then rotated about the axis of pin 253 until the required opticalalignment is achieved. In the preferred embodiment this is determinedwith a test fixture (not shown) referenced to an open aperture in disk31. Since the gear ratio between pinion 245 and disk 31 is chosen sothat for every 18 steps of motor 243 disk 31 is rotated from a positionwhere one lens or opening is aligned with axis 61 to a position wherethe immediately adjacent lens or opening is aligned with axis 61,alignment of an open aperture in disk 31 with optical axis 61 insuresalignment of the optical axes of all the strong sphere lenses with axis61. Once aligned, bracket attachment screws 273 and 275 are tightened.The gear mesh between teeth 269 and 271 is effectively unchanged sincethe displacement between the axis of drive shaft 265 and the axis of pin253 is minimal.

With the exception of the height of the bosses on which they aremounted, motor drive assemblies 19-25 are identical to motor driveassembly 17. Further, the structure and method of removing backlash andnoise, and achieving optical alignment for the lenses and opticalelements supported on disks 33-39 is the same as that used for disk 31.

As those skilled in the art will appreciate, the axes of the cylinderlenses supported on disks 33 and 37 must be rotatable about axis 61 inorder to orientate the cylinder axes so as to neutralize a patient'scylinder refractive error. The axes of the cross cylinder lensessupported on auxiliary disk 39 must also be rotatable about axis 61 andthis rotation synchronized with the rotation of the cylinder lenses. Thestructure for producing these required rotations is illustrated in FIGS.2 and 6. With reference to disk 33, this structure includes a pluralityof rotatable lens mounts, such as mount 53, bull gear 301, spool 83,drive tube 79, spool 91, cylinder axis drive gear 303, gear cluster 305,pinion 307 and stepping motor 309.

Lens mount 53 includes aperture 310 having a lens supporting seat (notshown), bearing surface 311, shoulder 313, gear teeth (not shown) andthree evenly spaced tabs, one of which is illustrated at 315. Asassembled, surface 311 bears against the surface of cylindrical opening317 of disk 33, with tab 315 hooking disk face 319 to thereby holdshoulder 313 against disk face 321. Bull gear 301 includes teeth (notshown), which mesh with the gear teeth (also not shown) on lens mount53, and a cylindrical aperture 323 in which is received surface 143 ofspool 83. Aperture 323 includes a key slot (not shown) which cooperateswith a key (also not shown) provided on spool 83 to prevent relativerotation therebetween. To prevent relative movement along axis 63, bullgear 301 is cemented to spool 83.

Stepping motor 309, preferably a North American Phillips K82401-P2 orequivalent, includes an integral frame bracket 331 which is secured viascrews, such as illustrated at 333, to motor support bracket 334 which,in turn, is secured to bosses, such as illustrated at 335 provided onback plate 13. Pinion 307, rigidly coupled to motor shaft 336, includesteeth 337 which mesh with teeth 339 on gear 341 of gear cluster 305.Teeth 343 of pinion 345 mesh with teeth 347 of axis drive gear 303. Asaxis drive gear 303 is both keyed and cemented to spool 91 and as spools91 and 83 are both keyed to drive tube 79, rotation of stepping motor309 rotates bull gear 301 which, in turn, rotates lens mount 53 and thecylinder lens (not shown) supported therein.

The rotatable lens mounts, such as illustrated at 57 and 59 of FIG. 2,provided on disks 37 and 39 are coupled to drive tube 79 via,respectively, bull gears 349 and 351, which are keyed and cemented to,respectively, spools 87 and 89. With this arrangement, rotation of motor309 produces simultaneous rotation of lens mounts 53, 57 and 59.

As those skilled in the art will appreciate, in addition to beingsimultaneously rotatable, the cylinder and cross cylinder lensessupported on disks 33, 37 and 39 must be prealigned and synchronized.Synchronization is accomplished by the gearing. All the lens mounts, asexemplified by mounts 53, 57 and 59, have the same number of gear teeth.Further, each of bull gears 301, 349 and 351 have the same number ofgear teeth. With reference to disk 33, the number of teeth on lens mount53 and bull gear 301 is chosen such that each complete rotation of bullgear 301, about axis 63 relative to disk 33, produces a multiple of 180degree rotations of lens mount 53. In the preferred embodiment, bullgear 301 is provided with 195 teeth and lens mount 53 with 39 teeth.With this arrangement, every complete rotation of bull gear 301 produces5 complete rotations of lens mount 53.

With reference to strong cylinder lens disk 33, the first step in thealignment procedure is to assemble and cement bull gear 301 to spool 83.With all the necessary rotatable lens mounts, such as mount 53,assembled thereto, disk 33 is assembled to spool 83. Next, the alignmentmark (not shown) provided on, for instance, mount 53 is aligned with thealignment mark (also not shown) provided on bull gear 301. This lattermark is aligned with the key slot provided in bull gear 301 to define anaxis which is perpendicular to axis 63. This procedure is repeated forall the lens mounts on disk 33. With the aid of a fixture, whichincludes a source of collimated light, the required cylinder lens isinserted in mount 53, such that the cylinder axis is perpendicular toaxis 63, and then cemented in place. This process is repeated for theremaining strong cylinder lenses.

With the weak cylinder lenses assembled to disk 37 and the crosscylinder assembled to disk 39 utilizing the procedure as set forthabove, spools 83, 87 and 89 and the structure supported thereon are,together with spools 81, 85 and 91 and the structure supported thereon,assembled on drive tube 79, as illustrated in FIG. 2.

Since stepping motor 309 is programmed to index only in integral steps,the axes of the cylinder lenses supported on disks 33 and 37 may, withinthe rotation increment of the lens mounts produced by one step of motor309, not be correct. To insure proper angular orientation, it isnecessary to provide for limited rotation of motor frame bracket 331,via motor support bracket 334 which attaches to back plate 13 andfunctions in the same manner as motor support bracket 241.

In contrast to prior art refractors in which each Risley prism mechanismis movable into and out of the optical axis, in the present inventionRisley prisms 353 and 355 are always positioned along axis 61.Accordingly, when not in use they must be orientated relative to eachother so that they are essentially equivalent to a glass plate withparallel faces. Prisms 353 and 355 and the supporting and rotatingmechanisms, illustrated in FIGS. 6 and 7, includes support assembly 357,motor assemblies 359 and 361 and gear trains 363 and 365.

Support assembly 357 includes prism mounts 367 and 369, base plate 371and cover plate 373. As illustrated in FIGS. 6 and 7, prism mount 367 isessentially a hollow cylindrical member having a lip 375 against whichprism 353 is seated. Prism mount 367 also includes faces 377 and 379 anda shoulder 381 on which are provided gear teeth (not shown). Similarly,prism mount 369 includes lip 383, faces 385 and 387 and shoulder 389having gear teeth thereon (also not shown). Prism mount 369 is receivedwithin opening 391 provided in base plate 371 with shoulder 389 seatingagainst shoulder 393. Prism mount 367 is received within opening 395 ofand held in place by cover plate 373. Internal shoulders provided oncover plate 373 (not shown) position and hold face 379 in bearingengagement with face 387. Cover plate 373 is secured to base plate 371by screws 397, 399 and 401. In turn, base plate 371 is secured tobosses, such as illustrated at 403 and 405, provided on back plate 13,via screws 407, 409 and 411.

Motor assembly 359 includes stepping motor 413, integral frame bracket415, drive shaft 417 and an alignment plate 419, which includes steppedopening 421. In the preferred embodiment, motor 413 is a North AmericanPhillips K82401-P2 or equivalent. Bracket 415 is secured to plate 419via clips (not shown), while plate 419 is secured to base plate 371 viascrews (not shown) which pass through enlarged openings therein (alsonot shown). In a like manner, motor assembly 361 includes stepping motor423, integral frame bracket 425, drive shaft 427 and alignment bracket429 having stepped opening 431.

Gear train 363 includes pinion 433, integrally formed gears 435 and 437and gear 439. Pinion 433, secured to drive shaft 417, is received withinstepped opening 421 and opening 441 provided in base plate 371, asillustrated in FIG. 7. Gears 435 and 437 are secured to plate 371 via aneccentric shoulder bolt 443. Gear 439, which is mounted on eccentricshoulder bolt 445, engages the gear teeth (not shown) provided onshoulder 381 of prism mount 367. As is evident from inspection of FIG.6, the teeth on shoulder 381 are exposed via cutout 446 in cover plate373. Similarly, gear train 365 includes pinion 447, integrally formedgears 449 and 451 and gear 453. Pinion 447, secured to drive shaft 427,is received within opening 455 provided in base plate 371 and steppedopening 431. Gears 449 and 451 are secured to plate 371 via eccentricshoulder bolt 457; gear 453, via eccentric shoulder bolt 459. The teeth(not shown) provided on shoulder 389 of prism mount 369 are exposed bycutout 461.

In the preferred embodiment, prism mount 367 is provided with 63 gearteeth, gear 439 with 20 teeth, gear 437 with 21 teeth, gear 435 with 50teeth and pinion 433 with 20 teeth. This results in a motor pinion toprism mount gear reduction of 7.5 to 1. Thus, a single step of steppingmotor 413, which produces a pinion rotation of 7.5 degrees, results in aone degree rotation of prism mount 367. The gear reduction betweenpinion 447 and prism mount 369 is also 7.5 to 1 so that each step ofmotor 423 produces a one degree rotation of prism mount 369. Inassembly, the backlash between gear 439 and prism mount 367 is adjustedby rotation of eccentric shoulder bolt 445. Similarly, the backlashbetween gear 439 and 437 is adjusted by rotation of shoulder bolt 443.With motor frame bracket 415 clamped to adjusting plate 419 via clips(not shown), the screws (not shown) which hold adjusting plate 419 tobase plate 371 are loosened and adjusting plate 419 is moved toward theaxis of gear 435 until the desired gear backlash adjustment between gear435 and pinion 433 is obtained. The screws clamping adjusting plate 419to base plate 371 are then tightened. The same process, utilizingadjustment plate 429, is used to adjust the gear backlash between pinion447 and gear 449.

To orient prisms 353 and 355 so that when not in use they effectivelyoptically neutralize each other, it is necessary to step motors 413 and423 until the optical bases of prisms 353 and 355 are 180 degrees apart.Since motors 413 and 423 are programmed to index only in integral steps,it may not be possible to orient prisms 353 and 355 accurately enough toneutralize each other completely without rotating at least one of motorframe brackets 415 and 425 to simulate a partial motor step. This isaccomplished by energizing motor 413 to utilize its magnetic detenteffect, unclamping the clips (not shown) which clamp motor frame bracket415 to adjusting plate 419 and then rotating bracket 415 until thedesired optical relationship is obtained between prisms 353 and 355. Asmotor shaft 417 is piloted in recess 421 of the adjusting plate 419,rotation of bracket 415 will not change the motor shaft center linelocation which, in turn, keeps the gear system backlash from changing.Alternately, this orientation of prisms 353 and 355 can be accomplishedby rotation of motor frame bracket 425 relative to adjusting plate 429.

Once the prisms 353 and 355 have been relatively located so as tocompletely neutralize each other, it is necessary to set the prism basedirection accurately. First, prisms 353 and 355 are each counter-rotated90 degrees so as to bring their bases into an alignment which willresult in the maximum additive prism power. The two prisms are thenrotated together, by use of stepping motors 413 and 423, until the basedirection of the prism pair is, for example, in the "base out"orientation. This measurement can be made with any of severalappropriate optical methods, such as projecting a laser beam throughprisms 353 and 355 and observing the direction of the deflection. If theexact "base out" direction cannot be obtained by identical integralsteps of the motors 413 and 423, it will be necessary, with the motorsenergized, to rotate both motor frame brackets 415 and 425 in unison,utilizing the same procedure used to initially orientate prisms 353 and355. The motor frame brackets 415 and 425 are then reclamped.

With the foregoing arrangement, Risley prisms 353 and 355 are controlledso that the full prism power range, in 0.50D steps, in the base out,base in, base up or base down, configuration can be effectivelyintroduced in optical path 61.

In contrast to prior art where the Jackson cross cylinder test isperformed by flipping the cross cylinder about the flip axis, in thepresent invention the test is performed by utilizing one of two sets of4 cross cylinders mounted on auxiliary disk 39. Each cross cylinder ismounted in a rotatable lens mount, such as illustrated at 59 in FIG. 2.As such mounts have the same number of gear teeth as mounts 53 and 57,as bull gear 351 is identical to bull gears 301 and 349, and becausebull gears 301, 349 and 351 are keyed to drive tube 79, rotation of thecross cylinders is synchronized with rotation of the cylinder lenses.Further, the cross cylinder lenses are prealigned, with the sametechnique utilized for aligning the cylinder lenses.

With reference to FIG. 8, auxiliary disk 39 includes set 461 of 0.50Dcross cylinders and a set 463 of 0.25D cross cylinder lenses. Set 461includes cross cylinder lenses 465, 467, 469 and 471. The orientation ofthe axes of the cylinder lens or lenses used to neutralize a patient'scylinder refractive error is represented by axis 473. As those skilledin the art will appreciate, the illustrated orientation of axis 473relative to lens 465 and optical axis 61 is arbitrary. As those skilledin the art will also appreciate, the illustrated orientation of axis 473relative to lenses 467-471 is for convenience of explanation only.Because lenses 467-471 are supported in rotatable mounts, which rotaterelative to disk 39 as disk 39 is rotated about axis 63, the correctorientation is determined by the gear ratio between these rotatable lensmounts and bull gear 351. For refining axis, 475 represents thedirection of the positive cylinder axis of lens 465. It is orientated at135 degrees relative to axis 473. Similarly, 477 represents thedirection of the negative cylinder axis, orientated at 45 degreesrelative to axis 473. With regard to lens 467, 479 represents thedirection of the positive cylinder axis; 481, the direction of thenegative cylinder axis. Relative to axis 473, axis 479 is orientated at45 degrees; axis 481, at 135 degrees. For refining power, lenses 469 and471 are utilized. In this case 483, which represents the direction ofthe positive cylinder axis of lens 469, is perpendicular to axis 473;485, the negative cylinder axis, is parallel to axis 473. For lens 471the positive cylinder axis is 487; the negative, 489.

In operation, to refine cylinder axis, auxiliary disk 39 is rotateduntil lens 465 is aligned with optical axis 61. As they are prealignedand synchronized with the cylinder lenses, axis 475 is orientated at 135degrees and axis 477 at 45 degrees relative to the axis of the cylinderlens or lenses which neutralize the patient's cylinder refractive error.To refine axis, stepping motor 491 of motor drive assembly 25 isenergized to rapidly rotate disk 39, via pinion 493, from the positionwhere lens 465 is aligned with axis 61 to the position where lens 467 isin alignment with axis 61. If each of lenses 465 and 467 produces anequal blur of the target being viewed, the orientation of the axis ofthe cylinder lens or lenses is proper. If one position is clearer thanthe other, the axis of the correcting cylinder lens or lenses is rotatedtoward, when the cylinder lens or lenses are positive, the one of crosscylinder axes 475 and 479 which produced better vision. Disk 39 is thenagain rotated to align the other of lenses 465 and 467 with axis 61 toagain determine if both lenses 465 and 467 produce an equal blur. Ifnot, the process is repeated until equal blurring is achieved.

To refine cylinder power, lenses 469 and 471 are utilized. As is evidentfrom inspection of FIG. 8, negative cylinder axis 485 is parallel to theorientation of the axis of the cylinder lens or lenses which neutralizethe patient's refractive error, while negative cylinder axis 489 isperpendicular thereto. If each of lenses 469 and 471 produces equalblurring, the power is correct. When the correcting cylinder is negativeand if vision is better with lens 469, when the minus axis 485 isparallel to the correcting cylinder axis, the power should be increasedand vice versa. This procedure is repeated until equal impairment ofvision is obtained with both lenses 469 and 471.

Lens set 463 includes lenses 495, 497, 499 and 501. Expect for the factthat they are all of 0.25D power, they are identical in function andorientation with lenses 465-471. Thus, lenses 495 and 497 have theiraxes orientated for refining cylinder axes, and lens 499 and 501 havetheir axes orientated for refining power.

Each of disks 31-39 is driven by, respectively, the stepping motors ofmotor drive assemblies 17-25. Similarly, stepping motor 309 drives therotatable lens mounts exemplified by mounts 53, 57 and 59 illustrated inFIG. 2, and stepping motors 413 and 423 rotate Risley prisms 353 and355. In total, each refractor half includes 8 stepping motors, each ofwhich must be driven by electrical pulses to incrementially rotate disks31-39, the rotatable lens mounts and Risley prisms 353 and 355, asnecessary to emulate the ophthalmic prescription required to correct thevision of the patient whose eyes are being examined. With reference toFIG. 9, these pulses are supplied by motor driver board 511 which, inturn, is controlled by computer board 513. The electronics also includesan encoder system 515, a display board 517, a key board 519, a printercontrol board 521, a printer driver board 523 and a thermal printer 525.

The heart of the electronics is computer board 513 which includesmirocprocessor chip 527, read only memory chips 529 and interruptdecoder logic system 531. In the preferred embodiment, microprocessor527 is an Intel 8048 or 8748 or equivalent, and memory chips 529 areIntel 8355 or 8755 or equivalent. As is evident from inspection of FIG.9, microprocessor 527 sends information to LED display 533 of displayboard 517, via printer control board 521. If the refractor is in thereset mode, wherein all of disks 31-39, lenses and Risley prisms 353 and355 are in their reset or zero position, display 533 will indicate this.Further, as disks 31-39 and the other optical components are rotated,microprocessor 527 sends information to display 533 to tell theinstrument operator which optical elements are positioned along opticalaxis 61 and, where appropriate, their orientation. Microprocessor 527also periodically checks to see if new instructions are coming from keyboard 519.

Key board 519 includes a set of OD refractor keys 535 and a set of OSrefractor keys 537. In the preferred embodiment, the keys are dome ormembrane switches. Set 535 includes plus and minus directional keys foreach of the motors which drive disks 31-37, motor 309, and motors 413and 423. In operation, when one of these keys is depressed, theassociated motor is energized for the number of steps required to movethe associated optical element from one operational position to theadjacent operational position. In addition, the directional keys formotors 309, 413 and 423 are coupled to a high speed interlock key which,when depressed, provides for high speed rotation. Rotation of strongsphere disk 31 is normally coupled to the rotation of weak sphere disk35. When the interlock key is depressed, the sphere directional keysmove disk 31 while disk 35 remains stationary. A duction key, forsimultaneous rotation of the Risley prisms of both refractor halves, isalso included. For the auxiliary elements supported on disk 39individual operation keys are provided. Set 537 includes a substantiallyidentical set of keys. Key board 519 also includes a reset key and keyswhich control printer control board 521. In the event thatmicroprocessor 527 is used not only to control both refractor halves butalso a compact refraction instrument, such as disclosed in U.S. Pat. No.4,395,097, key board 519 will also include a set of target and mode keys539 and additional reset keys.

In response to a key being depressed, a circuit in key board 519 isclosed and an electrical signal is sent to key board decoder 541 which,via a demultiplexer that senses which circuit was closed in key board519, sends a code to microprocessor 527. There is a different code foreach key and for each code there is an instruction in memory 529, whichinstruction results in signals being sent to motor driver board 511 torotate one or more motors a predetermined number of steps in aparticular direction. Microprocessor 527 also outputs new data todisplay 533 to indicate that the instruction has been carried out.

In order to hold disks 31-39, rotatable lens mounts such as illustratedat 53, and Risley prisms 353 and 355 in any given required position, allthe motors are constantly energized at, approximately, 1/4 power inorder to maintain the magnetic detent and, thus, keep the motors fromrotating. While the motors could be energized at full power, the lowerpower is preferred in order to reduce the size of the power supplyrequired and to reduce heat disipation. With this arrangement, motordriver board 511 includes, for each motor, a power up logic and a motordriver. With reference to FIG. 9, these are collectively designated 543and 545.

In the preferred embodiment, the signals from microprocessor 527 torotate, for instance, motor 243 a number of steps in a given directionare sent to both its motor driver and its power up logic. The motordriver keeps track of which two phases of motor 243 are on and which twophases are off. In response to the signals from microprocessor 527 torotate shaft 265 of motor 243, the motor driver sequentially changeswhich phases are on and which are off until the desired rotation isachieved. The power up logic includes the switching circuitry requiredto apply full power to, in this case, motor 243.

Printer control board 521, printer driver board 523 and thermal printer525 are for providing a printout of: (1) the retinoscopy finding; (2)the distance prescription; (3) the near prescription; and (4) thecomplete prescription of the patient whose eyes are being examined. Forthis purpose, printer control board 521 has its own microprocessor 547,preferably an Intel 8039, and its own read only memory 549, preferablyan Intel 8355 or 8755. Microprocessor 547 monitors the information sentto LED display 533 to determine if one of the print keys has beendepressed. If a print key has been depressed, microprocessor 547 sends asignal to microprocessor 527 to send the requested refraction data tomicroprocessor 547 for printing. Printer control board 521 also includesan RS-232-C input/output port that can be used to input key codes fromand output display data to a computer.

To rotate disk 31 from a position in which one strong sphere lens oropening is aligned with optical axis 61 to an immediately adjacentposition where another lens or opening is aligned with axis 61 requires18 steps of motor 243. As the gearing and motors are identical, 18 stepsare also required to rotate each of disks 33-39 from one alignmentposition to an immediately adjacent alignment position. Further, becauseof the gear reduction, each step of motor 309 produces a one degreerotation of lens mounts 53, 57 and 59. Finally, each step of motors 413and 423 rotates, respectively, prisms 353 and 355 one degree.

In order for microprocessor 527 to, for instance, rotate disk 31 from aposition where one lens or opening is aligned with axis 61 to a positionwhere another lens is aligned with axis 61, it is necessary to providedisk 31 with a reset or zero position from which all steps of motor 243are counted. This is accomplished by encoder system 515 and reflectiveblocks. With reference to FIG. 1, disk 31 is provided with a reflectiveblock 551 which cooperates with encoder 553, preferably a TexasInstruments TIL-139 or equivalent, positioned on comb assembly 73. Eachof disks 33-39 is provided with an identical reflective block (notshown). Further, as is evident from inspection of FIG. 4, comb assembly73 also includes optical encoders 555 and 557, for monitoring thepositions of disks 35 and 39. For monitoring the positions of disks 33and 37, comb assembly 75 is provided with two encoders, one of which isillustrated at 559 in FIG. 1.

Encoder 553, like all the encoders utilized, includes an LED and aphototransistor. Each phototransistor is coupled, and always sending ananalog signal, to a separate terminal in OD encoder board 561. Board 561also includes decoder logic which transforms these analog signals todigital signals, which are sent to interrupt decode logic system 531.For each encoder there is a separate terminal in logic 531. Inoperation, when the light emitted by the LED of encoder 553 is reflectedback to the phototransistor by the leading edge of block 551 as it movespast encoder 553, the analog signal sent by encoder 553 changes. Whenthe analog signal sent to board 561 reaches a threshold value, thedecoder logic changes the digital signal sent to logic 531. For eachmotor, logic 531 includes a latch, preferably a J-K master slaveflip-flop, which latches the rising edge of the change in signal fromthe decoder logic of board 561. In response to the latch beingtriggered, logic 531 sends an interrupt signal to microprocessor 527that one of the latches has been triggered, and a signal to memory 529which identifies which latch was triggered. If the rotation whichtriggered, for instance, encoder 553 was in response to a reset commandfrom key board 519, microprocessor 527 reads interrupt data latched onmemory 529 to find out which motor is associated with the interruptsignal received, and stops sending electrical pulses to, in the case ofdisk 31, motor 243. If the encoder is triggered in response to aninstruction other than a reset instruction, the signal transmitted fromlogic 531 to microprocessor 527 is ignored.

Since, for instance, stepping motor 243 is programmed to index only inintegral steps, it may be necessary, with motor 243 energized to hold itin one of its magnetic detent positions, to rotate motor support bracket241 to insure proper alignment of the lenses supported on disk 31 withoptical axis 61. To maintain this alignment, the electrical pulses whichdrive motor 243 must be interrupted so that motor 243 stops at thatdetent position where, in the case of disk 31, plano lens 60 is alignedwith axis 61. Misalignment occurs if motor 243 stops rotating one steptoo soon or too late. This is true for disks 33-39. For the rotatablelens mounts and Risley prism 353 and 355, failure of the associatedmotor to stop at the correct detent position results in angularmisorientation.

As those skilled in the art will appreciate, for motor 243 to stop atthe correct detent position, encoder 553 must be triggered prior to thecorrect detent position but subsequent to the immediately preceedingdetent position. Thus, the digital signal from encoder board 561 must bebetween two magnetic detent positions of motor 243 in order formicroprocessor 527 to stop motor 243 at the step at which disk 31 is atits reset position. In view of manufacturing tolerances, it is necessaryto be able to laterally adjust the position of each encoder to insurethat the required signal is transmitted with the desired 1/2 step. Withreference to FIG. 4, encoders 553, 555 and 557 are supported betweenforks 215 and 217 of comb assembly 73 on encoder support brackets 565,567 and 569. Bracket 565 is secured to forks 215 and 217 via screws 571and 573 which pass through slots 575 and 577 provided in tabs 579 and581. Slots 575 and 577 permit the necessary lateral adjustment ofencoder 553. Brackets 567 and 569 are identical in construction to, andadjusted in the same manner as bracket 565.

For Risley prisms 353 and 355 and the rotatable lens mounts, such as 53and 57 which support the strong and weak cylinder lenses, pairs ofencoders are required to insure that the associated motors stopprecisely at the reset or starting position. With reference to therotatable lens mounts, axis drive gear 303 is provided with 5 reflectiveblocks (not shown) equally angularly spaced about its periphery. Gear341 is provided with reflective block 583. One encoder (not shown) isassociated with gear 303; encoder 585, supported on post 587, with gear341. With this arrangement, only when the phototransistors of bothencoders are simultaneously above threshold and the resultant signalsANDed does encoder board 561 send the required digital signal tointerrupt logic system 531. With the gear ratios and with 5 blocks ongear 303, this occurs for every 720 degree rotation of lens mounts 53,57 and 59.

The encoder system for Risley prisms 353 and 355, illustrated in FIGS. 6and 7, includes reflective blocks 591, 593, 595 and 597 and 4 encoders,three of which are illustrated at 601, 603 and 605. Encoders 601 and 605are supported on adjustable brackets 607 and 609. With reference to, forinstance, Risley prism 353, because mount 367 rotates only one degreefor each step of motor 413, block 593 and encoder 603 cannot bepositioned accurately enough to insure that encoder 603 will always betriggered prior to the correct detent position but subsequent to theimmediately preceding detent position. In contrast to mount 367, gear435 rotates three degrees for each step of motor 413 and, hence, block591 and encoder 601 can be positioned to obtain the required accuracy.However, since gear 435 rotates 360 degrees for every 120 degreerotation of mount 367, encoder 601 cannot be used alone, but must becoupled with encoder 603. With this arrangement, only when thephototransistors of both encoders are simultaneously above threshold andthe resultant signals ANDed, does encoder board 561 send the requireddigital signal to logic 531.

As is evident from inspection of FIG. 9, encoder system 515 alsoincludes OS encoder board 611. Where microprocessor 527 is used not onlyto control both refractor halves but the compact refraction instrumentdisclosed in U.S. Pat. No. 4,395,097, an additional encoder board andset of encoders will be coupled to logic 531 and microprocessor 527.

Whereas the drawings and accompanying description have shown anddescribed the preferred embodiment of the present invention, it shouldbe apparent to those skilled in the art that various changes may be madein the form of the invention without affecting the scope thereof.

What I claim is:

1. In an ophthalmic instrument including a base and one or more opticalelements positionable along an optical axis:(a) a disk including aplurality of openings for supporting at least some of said opticalelements, said disk supported relative to said base for rotation aboutan axis from a position where one of said openings is aligned with saidoptical axis to at least one other position where another of saidopenings is aligned with said optical axis, one of said positions onsaid disk constituting a reset position when aligned with said opticalaxis; (b) a stepping motor having a plurality of detent positions and ashaft for driving said disk about said rotation axis, said motor beingin one of said detent positions when said disk is in said resetposition; (c) means for sending signals to said motor to rotate saiddisk; and (d) means for interrupting said signals to said motor beforethe magnetic detent position where said disk is in said reset positionbut subsequent to the immediately preceding detent position, said meansincluding a reflective block on said disk and an encoder securedrelative to said base.
 2. The instrument as set forth in claim 1,further including a support for said encoder, said support includingmeans which permit lateral adjustment of said encoder in the plane ofsaid disk.
 3. The instrument as set forth in claim 1, wherein said meansfor sending signals to said motor includes a microprocessor, and whereinsaid means for interrupting said signals to said motor includes decoderlogic means, coupled to said encoder, for transforming analog signals todigital signals, and a latch coupled to both said decoder logic meansand said microprocessor.
 4. The instrument as set forth in claim 3,wherein said encoder includes an LED and a phototransistor, said encodersending analog signals to said decoder logic means which signals changewhen the light emitted by said LED is reflected back to saidphototransistor by said block, said decoder logic means changing thedigital signal when said analog signals from said encoder reach athreshold value, said latch latching as a function of the change indigital signal and sending a signal to said microprocessor to stopsending signals to said motor.
 5. The instrument as set forth in claim4, wherein said latch latches on the rising edge of said change indigital signal.
 6. The instrument as set forth in claim 4 or 5, whereinsaid means for sending signals to said motor includes motor driver meansfor keeping track of which phases of said motor are on and which phasesare off and for sequentially changing which phases are on and which areoff.
 7. The instrument as set forth in claim 6, wherein said motor isenergized at less than full power in order to maintain its magneticdetent position when said disk in one of its positions, and wherein saidmeans for sending signals to said motor to rotate said disk includes aswitching circuit to apply full power to said motor to rotate said motorshaft.
 8. In an ophthalmic instrument including a base and a pluralityof optical elements positionable along an optical axis:(a) a pluralityof disks, each of said disks including a plurality of openings forsupporting said optical elements, each of said disks supported relativeto said base for rotation about an axis from a position where one ofsaid openings is aligned with said optical axis to at least one otherposition where another of said openings is aligned with said opticalaxis, one of said positions on each of said disks constituting a resetposition when aligned with said optical axis; (b) a like plurality ofstepping motors having shafts for driving said disks about said rotationaxis, each of said motors having a plurality of detent positions, eachof said motors being in one of said detent positions when thecorresponding one of said disks is in reset position; (c) means forsending signals to each of said motors to selectively rotate each ofsaid disks; and (d) means for interrupting said signals to each of saidmotors before the magnetic detent position where said disk is in saidreset position but subsequent to the immediately preceding detentposition, said means including a reflective block positioned on each ofsaid disks and a corresponding encoder secured relative to said base. 9.The instrument as set forth in claim 8, wherein said means for sendingsignals to said motor includes a microprocessor, and wherein said meansfor interrupting said signals includes, for each of said motors: decoderlogic means, coupled to said encoder, for transforming analog signals todigital signals, and a latch coupled to both said decoder logic meansand said microprocessor.
 10. The instrument as set forth in claim 9,wherein each of said encoders includes an LED and a phototransistor,each of said encoders sending analog signals to said correspondingdecoder logic means, which signals change when the light emitted by saidLED is reflected back to said phototransistor by said correspondingblock, each of said decoder logic means changing the digital signal whensaid analog signals from said corresponding encoder reach a thresholdvalue, said corresponding latch latching as a function is the change indigital signal and sending a signal to said microprocessor to stopsending signals to said corresponding motor.
 11. The instrument as setforth in claim 10, wherein each of said latches latches on the risingedge of said change in digital signal.
 12. The instrument as set forthin claim 10 or 11, further including memory means coupled between saidmicroprocessor and said latches, each of said latches including meansresponsive to said latch latching for sending an interrupt signal tosaid microprocessor that one of said latches has latched, each of saidlatches also including means responsive to said latch latching forsending a signal to said memory which identifies which of said latcheswas triggered.
 13. The instrument as set forth in claim 12, wherein saidmeans for sending signals to said motors include, for each of saidmotors: motor driver means for keeping track of which phases of saidmotor are on and which phases are off and for sequentially changingwhich phases are on and which are off.
 14. The instrument as set forthin claim 13, wherein each of said motors is energized at less than fullpower in order to maintain its magnetic detent position when saidcorresponding disk is in one of its positions, and wherein said meansfor sending signals to each of said motors to rotate said correspondingdisk includes, for each of said motors, a switching circuit to applyfull power to said motor to rotate said motor shaft.